Substrate-supported phospholipid bilayers are of interest for many reasons. Firstly, these bilayers represent a convenient and versatile model of cellular membranes. Starting with the studies of McConnell and coworkers,1,2 it is now well documented that phospholipid vesicles fuse spontaneously into planar membranes when incubated on treated surfaces.3,4 In such planar assemblies the lipids are mobile as in vesicles4 and are suitable for incorporation of transmembrane proteins.2 Secondly, many inorganic substrates can be functionalized by self-assembling lipid bilayers on the surfaces.3 This substrate biofunctionalization is considered by many authors as one of the most attractive ways for building hybrid nanoscale devices and combinatorial assays to screen protein and phospholipid libraries for specific membrane-protein interactions.3,4 Recently, Boxer and coworkers described spatially addressable libraries of chemically distinct phospholipids that can be used for such a screening and also for studying the mechanisms of such important intracellular processes as protein trafficking and cell activation.4,5 Another potential use of the phospholipid arrays is for pattering of membrane proteins that would not require covalent attachment of proteins to the surface.
While several different approaches for building substrate-supported membranes are described in the literature,3,4,6 essentially all of them are based on a planar design in which phopsholipids are patterned/deposited on mostly essentially flat surfaces (see FIG. 1). Planar bilayers can be assembled by covalently attaching the inner monolayer to the substrate. These substrate-supported bilayers are very stable but lack some phospholipid mobility that can be achieved when the entire bilayer is suspended from the substrate on an ultrathin 5-15 Å water and/or polymer layer. The latter planar bilayer exhibits a greater resemblance with cellular membranes.4 In addition, suspended bilayers are more suitable for incorporation of membrane peptides and proteins because there is less steric congestion on the substrate side than for covalently-attached bilayers. Spanning planar bilayers over the outside surface of anodically etched porous alumina might be helpful for decreasing the congestion for those regions of the bilayer that lay above the pores.6 
Although planar phospholipid membranes are ideally suited for surface spectroscopy and imaging,4,5,7 this technology has some limitations for building robust arrays of biosensors. Particularly, the planar lipid assemblies and protein-on-a-chip devices are very fragile because the entire surface of the chip is exposed to the environment. Even any minor mechanical perturbation or contamination of the surface such as, e.g., an accidental touching or scratching, would be of a catastrophic consequence to the fragile phospholipid assembly. Also, in order to maintain the phospholipid order the surface of such an array should be kept hydrated and special care should be taken to keep this type of biochip from drying. In addition, the maximum number of lipid molecules and membrane proteins that could be deposited on such a chip is limited by the area of the planar supporting surface minus the total area of barriers.